Overvoltage surge arrester with improved voltage grading circuit

ABSTRACT

An overvoltage surge arrester for relatively high voltage applications is provided with a voltage grading circuit comprising a relatively low exponent resistor in parallel with a relatively high exponent resistor, for preventing spurious operation of the arrester under conditions in which the outside surface of the porcelain housing becomes contaminated with a conductive film. A linear resistor is additionally connected in series with the high exponent non-linear resistor of the above circuit to prevent excessively high current in the high exponent resistor near the sparkover voltage of the arrester.

United States Patent 1 Kresge OVERVOLTAGE SURGE ARRESTER WITH IMPROVED VOLTAGE GRADING CIRCUIT [75] Inventor: James S. Kresge, Pittsfield, Mass. [73] Assignee: General Electric Company [22] Filed: Jan. 16, 1974 [21] Appl. No.: 433,656

[52] U.S. Cl 317/68, 315/36, 317/70 [51] Int. Cl. H02h 9/06 [58] Field of Search 315/36; 317/68, 69, 70

[56] ReterencesCited UNITED STATES PATENTS 3,091,721 5/1963 Yost 317/70 3,313,978 4/1967 Miller 315/36 3,611,044 10/1971 Osterhout et al. 317/70 Jan. 7, 1975 Primary Examiner-James D. Trammell Attorney, Agent, or Firm-Volker R. Ulbrich [57] ABSTRACT An overvoltage surge arrester for relatively high voltage applications is provided with a voltage grading circuit comprising a relatively low exponent resistor in parallel with a relatively high exponent resistor, for preventing spurious operation of the arrester under conditions in which the outside surface of the porcelain housing becomes contaminated with a conductive film. A linear resistor is additionally connected in series with the high exponent non-linear resistor of the above circuit to prevent excessively high current in the high exponent resistor near the sparkover voltage of the arrester.

8 Claims, 6 Drawing Figures minimu W5 3.859569 SHEET 10F 3 PR/OR 31y mimsnm 3.859.569

' SHEETEUF a III PATENTEDJAH 11% 3,859 569 SHEET 30? 3 C'UIPRENTINflMPEKES 33,467,936 issued-Sept. 16,;19s9rt'o1euasser-" I 3,510,726 issued May 5, l-9 70f.to J1 Eglrl'ardjer. 1 ..3,6 8'3,23'4issued Aug. 8';.l972fto; A. 'Rodewald 2 2,688,715 issued Sept. 1,1954 to S. AL-Yorts-et al-,*.- Contaminationof the. porcelain may'cause'the afrrester- 'to fail by 'sparki'ngo've'r; at the operating voltage, rather than atrhe appropriate higher sparkovervoltiage.Such firequently results in destruction of I I the arrester.

" For an unde'rstanding'of the above d'e'sc'ribed mode it is useful to consider thealternating current" leakage due to capacitive coupling of thearrester. T "facilitate thediscussio'n of such capacitive leakage,

.l there is shown in FIG. 1 of the drawings aschematic representation of a fragment of a prior art arrester 10. The arrester 10 includes three of a numberof-arrester modules 12 inside a housing cylinder 14 provided with an upper end cap 16. Each module 12 is outlined by a OVERVOLTAGE some ARliESTERsfWlTH VED VOLTAGEGRADIN G"ClRCUlT BACKGROUND) OF THE INVENTION The present-invention relates generally to electrical overvoltage, surge arresters and relates particularly to, but is no'flimited to, such arresters for hse at relatively high voltages and which'are provided with a plurality of spark gaps electrically conn'e'cted in series.

An overvoltage surge' arrester for relatively high alternatingcurrentvoltages of 3 kv (kilovolts) or higher v the lowermost module and is connected to ground.

Each arrestermoduleincludes one or more electrode gaps connected in series with one or'more nonlinear resistance current=limiting elements, or, valve blocks. A

;avoid confusion of the coupling capacitances with actual capacitors which are sometimes included in an arrester.

Theifirst coupling components C C C of the first, second, and third modules, respectively, beginning withthe uppermost, are the capacitance of the module 12 itself, including, for instance, the capacitance between the electrodes of the gap section 18.

The second coupling components C C C of each respective module 12 are the capacitance due to non-linear grading resistor iscommonly connected in 7 parallel with the gapof each module to maintain a uniform voltage distribution among the modules. Additional components may be connected in or among the modules to further. enhance the operation of the artester," Theindividual' rrlodules,aredesigned to normally have acrossthem an 'operatingvoltage.TEach module also has a characteristic ,spa'rl over -voltage/f somewhat above theoperatingvoltage, atmwhic h the gap" sparks'over to initiate anactive operation. of the arrester. Typically, the sparkover-voltage is on the order of 1.7 times the operating voltage, The operating volt=.

age and the sparkover voltage ofan' entire arrester are. r

erratic operation failure simply the sum of the respective such voltages of the i modules-which are connectedin series in the arresterj Thus, thearrester is designed to have an operating voltfagcj,v equal 'to'theIn'ormal. line-to-ground voltage of the line which isto be 'conne'cted-tothe 'ar'rester. 'A-se'r iou-s problem with hig-h voltage arrest'ers of: the

type-described'above canariseiv'vhen the outside surfaceof the porcelain housing becomes' contiaminated with an electricallyconductive film ofi jfor exampleisalt spray or wettedfcement dust. VariouseffeCtsof'such'. contaminationare described for exampleii' e folljo ing ULS; PaLNosz:

.;r,ents'.th rough capacitances C21, C22. C

the couplingof the internal parts of the arrester 10 with the housing 14..

The third coupling components C C C of each respective module 12 arethe capacitance of the porcelain housingc'ylinder 14' itself.

The fourth couplin'g'components C C C of each respectivemodule 12 are the capacitance due to coupling of the porcelain housing cylinder 14 to ground: I

During normal operation of the arrester 10, the voltage across C and therefore across the gap section 18 of the first module 12, is greater than the voltage across cg; because C and the grading resistor 22 of the first module 12 must carry all the capacitive leakage and the normal grading currents'for all the other modules 12 down the line. The grading resistors 22 are typically chosen so that during normal operation the grading current through them is much larger than the capacitive leakage current. Thus, the voltage across a given gap section 18,is held by the resistors 22 at very nearly the same value'as, across every other gap section 18. At normal line voltages, the total capacitive leakage current is typically on the order of, for instance, 0.01 milliamperesdwhile the grading current through the grading resistors 22 is typically on the order of l milliampere.

When the surface of the arrester housing 14 is contaminatedwith a conducting contaminant such as salt,

cementdusL fly ash etc. and the contaminant is wetted so' that it becomes capable of conducting current and initiating discharges on portions of the surface, the cur- .increase considerably. Thereason for the increase is that the voltagedistributioh on the porcelain surface becomes extremely-nonuniform.at times fromrapidly changing nonuniformfsurface conductivity-of the contaminant film due'touneven'w'etting andfr'om' the effects of discharges on the wettingpatternsj Since the current in a 1 capacitor is proportional to {the rate of change ofvoltage across it, such rapidly changing voltage distribu-'-;

tions result sporadically in substantially increased capacitive leakage currents being forced through the grading resistors 22,-particularly those nearest the line voltage; Peak leakage currents of tens of milliamperes or higher may be coupled into the arrester 10 undersev erely contaminated'conditions. Such relatively large sporadic capacitive leakage currents through the grad- 'ing resistors '22 can result in a voltage drop across'the grading resistors 22 high enough to cause a spurious application'Ser. No. 433,655, entitled ffOvrvoltage Surge Arrester With Improved Voltage GradingCircuit, filed in the name of E. C. Sak'shaug on the same date and assigned to the same assignee as is the'present application. The'term exponent as used herein refers to the value'of the current-voltage characteristic exponent n of the voltage in the current-voltage relationship for a resistor given by I=I(V", where I represents the current through the resistor, K represents a constant, and V represents the voltage across the resistor. When an arresterprovided with such low and high exponent parallel grading resistors is operating in a steady state at operating voltage, the low exponent grading resistors FIG'." I is asidesectional view of afragment prior 'art arrester and including a schematic representation of 1 an'electrical circuit therein.

allow a gradingcurrent on the order of milliamperes to I flow through them", such that any capacitance effect. in the absence of contamination on external surface of the housing is not significant at line voltage frequencies. Consequently, the voltage is relatively uniformily graded over the gap section. The current through the high exponent grading resistors is very small, in the order of only microamperes. This is not enough current to present a significant risk of instability failure of relatively stable compositions of such high exponent resistors. When contamination of the arrester results in sub- 'stantially increased capacitive leakage current to be sporadically forced through the grading resistors, the bulk of this current is passed through the high exponent resistors. Because of the high exponent of the resistors, this current can be passed'without sparking over the gap section, since the resistive voltage drop can be maintained at well below the sparkover voltage.

A high current problem exists with the above parallel low and high exponent grading resistor arrangement when the desired sparkover voltage is made relatively high as compared to the operating voltage. If the high exponent gradingresistor is chosen to pass a current on the order of a milliampere at'the rated voltage, as is desired for it tobe effective as described above, then the current will be unreasonably high as sparkover is approached. Inaddition, sincein most modern multi-gap arresters at least'some of the gaps'are cascade gaps which are set to have a sparkover substantially higher than other gaps which'are set to trigger the total arrester sparkover, this high current problem is even more serious for the high exponent grading resistors paralleling these so-called cascade-gaps. The'high currents passed through the high exponent grading resistors just prior to and during sparkover of the arrester resulting from an overvoltage surge can be so large as to cause instantaneous'heating damage to the high exponent resistor material. such heating damage can lead to failure of the high exponent resistor.

SUMMARY or THE INVENTION The novel arrester comprises a gap having in parallel with it both a relatively low exponent grading resistor and a relatively highexponent grading resistor, connected 'in parallel relationship to one another. A' sub- FIG. 2 is aside sectional view of a fragment'of an overvoltagesurge arrester in accordance with the preferred embodiment of'the invention. a

FIG. 3 is a side view of an arrester module of the arrester of FIG. 2 seen at with respect to the view of FIG. 2.

FIG. 4 is a partly sectioned perspective view of aportion of the module of FIG. 3.

' FIG. 5 is a side, sectional, partly schematic view of a fragment of the arrester of FIG. 2.

FIG. 6 is a graph illustrating the current voltage characteristics of the grading resistors. of the arrester module of FIG. 2

A preferred embodiment of the novel arrester is shown in FIG. 2. The arrester 24 has a porcelain housing 26 sealed at both ends by metalend caps, not shown. Inside the housing 26 and clampedbetween the end caps is-a stack of individual-arrester modules 28 paired side by side, only a pair of which are shown entirely in the FIG. 2. Another view of the modules 28 at 90 orientation with respect to the FIG. 2 is shown in FIG. '3. A perspective view of a single module 28 is shown in FIG. 4. All the modules 28 of the arrester 24 are similar and have a 6 kv rating, meaning that they are designed to be subjected to an individual operation voltage of about 4.8 kv. Like reference numerals are used to identify like members of the modules 28 of FIGS. 2,3 and '4.

Referring now to FIGS. 2, 3 and 4, each module'28 includes a gap section, or unit 30 contacted on each of its-faces'by" a valve block 32. Connected electrically in parallel with the series of the gap-section 30 and the valve blocks 32 are a high exponent grading resistor 34 and-a low-exponent grading resistor 36. The modules 28 are series-stacked in pairs whichare clamped on insulatingspacers 38 between metal supports plates 40 facing in opposite directions for connection in series as a pair by a diagonal vmetal strap 42 extending between two thin metal contact plates 44, each located between the spacer 38"and the 'valve block 32. The gap electrodes of the gap unit 30 are located inside ceramic supporting discs of the gap unit 30 and are not shown (centimeters) long, and I cm in diameter and has a current-voltage characteristic exponent of about 4.5.

'a'rni 48lattached to the contact plate 44,, A linear .resistor 50 is connected between the clip-400i] the injsulatingarmf48 and the contact plate 44. The high exponent grading resistor 34' is about cm long, about 1 1.6 cm in diameter and'has a current-voltage characteristic exponent ofabou't 45'. The linear resistor 50 has a resistanc'evalue of about 1,000 ing of about 2 watts."

ohms and a'power rat- There is shown in'FIG'. 5 of the drawings a pa'rtiallly schematic representation of the electrical elements of the arrester 24 and the capacitive coupling alternating current leakage components associated with contamil nation of the arrester housing 26. The capacitive coupling components are represented'by the same symbols used in the discussion above relating to FIG. 1, since the coupling components of FIG. 5 are analagous to those of the earlier discussion. The. first'arrester mod ;ule 28 at the top and those'following is outlined by dashed lines 51. The first module 28 includes a'valve block 32 electrically conneeted to each side ofthe gap rier only at or above the rated voltage. Since the sparkover voltage of the gap section 28 is generally at least -125 percent of'the rated voltage, sparkover of the gap section 28 by spurious leakage currents in the grading resistors 34, 36 is prevented by the high exponent resistor 34. It can be seen from the broken line curve 56 that just below the sparkover voltage, which in this instance is taken to be about 135 percent the rated voltage, the voltage across the linear resistor rapidly becomes' significant to limit the current upturn in the high exponent resistor 34 connected in series with it.

GENERAL CONSIDERATIONS While the exponent of the relatively high exponent substantially greater than the exponent of the low expo- 'nent material to provide thebenefits of the present invention. A substantially greater exponent" is taken to mean that the exponent is greater by more than the ex- ;tent to which the exponent normally variesfor a given 25. section 30. Connected in parallel with theremote sides the high exponent grading resistor 34and the linear resistor 50. l 4

A graphic representation of the current-voltage char acteristics of the low exponent, high exponent, and -lin ear grading resistors 36, 34, 50 of the arrester 10 of the preferred embodiment is presented roughly in FIG. 6of the drawings. The abscissa of the graphcorresp'onds to the current in amperes'throughv the resistor-plotted on material in the production'process. Moreover, the low exponentmaterialis not limited to silicon carbide, but

may be of various nonlinear resistance materials which .are presentlyused, or could presumably be used in an arr'ester for that purpose. For that matter, the relatively lowexponent material could in fact be linear, thus havingan exponent of unity. For practicalreasons, hownected in parallel with a' gap includes arrangements in whichvalveblocksor various other elements, including a logarithmic scale, while the ordinate corresponds ,to

the rated voltage of the arrester module 28; The oper-' ating voltage is, as was mentioned earlier, generally otherfgaps, areadditionally connected. in series with the gapandiin parallel with the resistor;

f r "The, resistance value and continuous power dissipaabout 80 percent of the rated voltage. -T he dashed.

curve 52 shows the behavior of the low exponent resis} tor 36. As the currentrises, the voltage across theresis- I tor 36 also rises at a relatively rapid'rate. The solid" curve 54 shows the behavior of the high exponent resis tor 34. The voltage rises relatively slowly with increasing current. The broken line curve 56 illustrates the manner in which the current through the high exponent f resistor 34 is modified by the addition of thelinear re sistor 50. It can be seen from the two curves 52,54 that with the nonlinear resistors 34, 36 connected in parallel f so that they see the same voltage, almost all the'current is carried by the low exponent resistor 36 at the operat-i ing voltage of 80 percent rating and the effect of. the linear resistor is insignificant at this voltage. As the current increases so that the voltage rises to 100 percent the rated voltage, the current becomes evenly di vided between the low and high exponent resistors 36, 34. The major portion of higher currents than those at 100 percent rated voltage is carried by the high exponent resistor 34. Thus, the resultant curve for the paral-' t'ion capability'o f thelinear resistor should be chosen to. provide the desired current limiting action for the particularsparkover voltage. of thegap in-question, and

is not particularly'critical. For sparkover voltages on the'orderofkilovolts, aresistance on the order of 0.5 to 5 kiloohms iscon'sideredsufficient forvarious compositions of high exponent resistors. It. is generally adequate to use as the linear resistor a carbon composition bulk type resistor having sufficient mass to absorb the "energy dissipated within it during an overvolt'age event wi'thouft' overheating.

The linearxresistor in series witht he high exponent resistor-need not'be absolutely linear, solong as it is substantially linear relativeto the high exponentresistorL' Asa practical matter, the linear resistor should i g have a current-voltage exponent ofless than two. Thus,

the term substantiallylinearas'used herein to de;

- semblies, and the gap assemblies may-be all of the same type, such'as a simple gap, or of'a complex type, such as a current limiting typejThey may also be a mixture of different types ofgap assemblies'connected together in series, parallel, or combinations thereof. The important consideration is that the gap section be essentially 7 a voltage-sensitive switch which closes'very suddenly at a predetermined voltage higher. than the voltageto which itiis subjected during normal operation at the operating voltage of the arrester..

.lclaim: a I 1. An electrical overvoltage including:

.a housing comprising at least two conductive terminal members spaced apart by a hollow insulating member; v

surge arrester of the type a spark gap section disposed inside said housing, said spark gap section comprising at least one spark gap assembly electrically connected in series with said terminal members;

a first grading resistor of a first nonlinear resistance material electrically connected in parallel with said spark gap section, the degree of nonlinearity of said first material being indicated by a first numerical exponent for the voltage in an equation describing the general current-voltage characteristics of said first materiaL and a second grading resistor of a second nonlinear resistance material electrically connected in parallel with said spark gap section and with said first grading resistor, the degree of nonlinearity of said second material being indicated by a second numerical exponent for the voltage in an equation describ-' ing the general current-voltage characteristics of said second material,

said-first exponent being substantially greater. than said'second. exponent; wherein the 1 improvement comprises; i a substantially linear resistor electrically connected in series with said first grading resistor. 2. The arrester defined in claim 1 and wherein said first exponent is at least'twice the magnitude of said second exponent. v

3. The arrester definedin claim 2 wherein said first exponent isabout ten times as great as said second exponent and said linear resistor has a resistance value on the order of at least several hundred ohms and a current-voltage characteristic exponent of less than two.

4. The arrester defined in claim 3 wherein said first material is substantiallya metal oxide.

5. The arrester defined in claim 4 wherein said first material is substantially zinc oxide and said linear resistor has a resistance value of about 1,000 ohms.

6. The arrester defined in claim 5 wherein said second material is substantially silicon carbide.

7. The arrester defined in claim 6 comprising at least one nonlinear resistance valve block element electrically connected in series between said spark gap and one of said terminal members. 7

8. The arrester defined in claim 7 wherein said spark gap section comprises a plurality of said spark gap assemblies electrically connected in series between said terminal members. 

1. An electrical overvoltage surge arrester of the type including: a housing comprising at least two conductive terminal members spaced apart by a hollow insulating member; a spark gap section disposed inside said housing, said spark gap section comprising at least one spark gap assembly electrically connected in series with said terminal members; a first grading resistor of a first nonlinear resistance material electrically connected in parallel with said spark gap section, the degree of nonlinearity of said first material being indicated by a first numerical exponent for the voltage in an equation describing the general current-voltage characteristics of said first material, and a second grading resistor of a second nonlinear resistance material electrically connected in parallel with said spark gap section and with said first grading resistor, the degree of nonlinearity of said second material being indicated by a second numerical exponent for the voltage in an equation describing the general current-voltage characteristics of said second material, said first exponent being substantially greater than said second exponent, wherein the improvement comprises; a substantially linear resistor electrically connected in series with said first grading resistor.
 2. The arrester defined in claim 1 and wherein said first exponent is at least twice the magnitude of said second exponent.
 3. The arrester defined in claim 2 wherein said first exponent is about ten times as great as said second exponent and said linear resistor has a resistance value on the order of at least several hundred ohms and a current-voltage characteristic exponent of less than two.
 4. The arrester defined in claim 3 wherein said first material is substantially a metal oxide.
 5. The arrester defined in claim 4 wherein said first material is substantially zinc oxide and said linear resistor has a resistance value of about 1,000 ohms.
 6. The arrester defined in claim 5 wherein said second material is substantially silicon carbide.
 7. The arrester defined in claim 6 comprising at least one nonlinear resistance valve block element electrically connected in series between said spark gap and one of said terminal members.
 8. The arrester defined in claim 7 wherein said spark gap section comprises a plurality of said spark gap assemblies electrically connected in series between said terminal members. 